System And Method For Detecting And Isolating A Remote Loop

ABSTRACT

A system and method are provided for enabling a first network to detect a loop in a second network connected thereto. The first network runs a first instance of a Spanning Tree Protocol and the second network runs either a different instance or no instance. The method includes sending a Remote Loop Detection Packet (“RLDP”) from the ports in bridges of the first network which are connected to the second network. The RLDP includes identifiers such as the source bridge, port and VLAN. The system and method further includes checking for receipt of the RLDP on the same bridge which sent the RLDP. If such a receipt occurs, a loop is detected and one of the ports of the receiving/sending bridge is blocked.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 12/466,363, filed on May14, 2009, which is a continuation of prior U.S. patent application Ser.No. 10/632,591, entitled “System and Method for Detecting and Isolatinga Remote Loop”, filed on Aug. 1, 2003, now U.S. Pat. No. 7,558,205.

FIELD OF THE INVENTION

The invention relates to network configuration protocols, and, moreparticularly, to protocols which enable remote loop detection and allowfor isolation of those remote loops.

BACKGROUND OF THE INVENTION

A computer network typically comprises a plurality of interconnecteddevices, These devices include any network device, such as a server orend station, that transmits or receives data frames. A common type ofcomputer network is a local area network (“LAN”) which typically refersto a privately owned network within a single building or campus. LANsmay employ a data communication protocol, such as Ethernet or tokenring, that defines the functions performed by the data link and physicallayers of a communications architecture in the LAN. In many instances,several LANs are interconnected by point-to-point links, microwavetransceivers, satellite hook-ups, etc. to form a wide area network(“WAN”), that may span an entire country or continent.

One or more intermediate network devices are often used to couple LANstogether and allow the corresponding entities to exchange information.For example, a bridge may be used to provide a bridging function betweentwo or more LANs. Alternatively, a switch may be utilized to provide aswitching function for transferring information among a plurality ofLANs or end stations. In effect, a switch is a bridge among more than 2networks or entities. The terms “bridge” and “switch” will be usedinterchangeably throughout this description. Bridges and switches aretypically devices that operate at the Data Link layer (“layer 2”) of theOpen Systems Interconnection (“OSI”) model. Their operation is definedin the American National Standards Institute (“ANSI”) Institute ofElectrical and Electronics Engineers (“IEEE”) 802.1D standard. A copy ofthe ANSI/IEEE Standard 802.1D, 1998 Edition, is incorporated byreferenced herein in its entirety.

Telecommunication traffic among network devices is divided into sevenlayers under the OSI model and the layers themselves split into twogroups. The upper four layers are used whenever a message passes to orfrom a user. The lower three layers are used when any message passesthrough the host computer, whereas messages intended for the receivingcomputer pass to the upper four layers. “Layer 2” refers to thedata-link layer, which provides synchronization for the physical leveland furnishes transmission protocol knowledge and management.

Networks may be designed using a plurality of distinct topologies—thatis the entities in the network may be coupled together in many differentways. Referring to FIGS. 1-3, there is shown different examples of“ring” topologies. A ring topology is a network configuration formedwhen “Layer 2” bridges are placed in a circular fashion with each bridgehaving two and only two ports belonging to a specific ring. FIG. 1 showsa single ring 50 having bridges 52 connected by paths 54. Each bridge 52in ring 50 in FIG. 1 has two ports 52 a and 52 b belonging to the ring.FIG. 2 shows two adjacent rings, 50 a and 50 b, with a single bridge 56having two ports 56 a, 56 b belonging to each ring.

In FIGS. 1 and 2, no paths or bridges are shared among rings. In FIG. 3two rings 50 c and 50 d are connected and share two bridges 58, 60.Bridge 58 has two ports 58 a and 58 b which each uniquely belong to onlyone ring, rings 50 c and 50 d respectively. Bridge 58 also has one port58 c connected to a path which is shared by both rings 50 c and 50 d. Ifrings are assigned different priority levels, a port such as 58 cconnected to the shared link assumes the priority value of the higherpriority ring, and ports 58 a and 58 b in shared bridge 58 and port 60 ain bridge 60 connected to the lower priority ring are deemed to becustomer (or lower priority) ports. The use of a shared link betweenshared bridges 58, 60 allows for the connection of rings and the growthof a larger network from smaller ring components; however, the sharedlink also presents difficulties since its failure affects both rings 50c and 50 d.

Ring topologies shown in FIGS. 1-3 present Layer 2 traffic loopingproblems. As illustrated in FIG. 4, in a single ring topology, datatraffic can circulate around in either direction past their originationand thus create repetition of messages. For example, data traffic mayoriginate in bridge 51, travel counter-clockwise in the ring, passbridge 57 and return to bridge 51; this is called a loop. Loops arehighly undesirable because data frames may traverse the loopsindefinitely. Furthermore, because switches and bridges replicate (i.e.,flood) frames whose destination port is unknown or which are directed tobroadcast or multicast addresses, the existence of loops may cause aproliferation of data frames that effectively overwhelms the network.

To prevent looping, one of the paths in the ring is blocked, as shown inFIG. 4, by blocking data traffic in one of the ring ports—in this case,either port 51 a or 57 a. The port is deemed to be in a “blocking”state, in which it does not learn or forward incoming or outgoingtraffic.

A network may be segregated into a series of logical network segments.For example, any number of physical ports of a particular switch may beassociated with any number of other ports by using a virtual local areanetwork (“VLAN”) arrangement that virtually associates the ports with aparticular VLAN designation. Multiple ports may thus form a VLAN eventhough other ports may be physically disposed between these ports.

The VLAN designation for each local port is stored in a memory portionof the switch such that every time a message is received by the switchon a local port the VLAN designation of that port is associated with themessage. Association is accomplished by a flow processing element whichlooks up the VLAN designation in the memory portion based on the localport where the message originated.

Most networks include redundant communications paths so that a failureof any given link or device does not isolate any portion of the network.For example, in the ring networks shown in FIGS. 1-4, if communicationis blocked preventing data from flowing counter-clockwise, the data maystill reach its destination by moving counter-clockwise. The existenceof redundant links, however, may also cause the formation of loopswithin the network.

To avoid the formation of loops, many network devices execute a“spanning tree algorithm” that allows the network devices to calculatean active network topology which is loop-free (e.g. has a needed numberof ports blocked) and yet connects every element in every VLAN withinthe network. The IEEE 802.1D standard defines a spanning tree protocol(“STP”) to be executed by 802.1D compatible devices (e.g., bridges,switches, and so forth). In the STP, Bridge Protocol Data Units(“BPDUs”) are sent around the network and are used to calculate the loopfree network technology.

Other available protocols include that shown and described in nowpending NETWORK CONFIGURATION PROTOCOL AND METHOD FOR RAPID TRAFFICRECOVERY AND LOOP AVOIDANCE IN RING TOPOLOGIES, filed Mar. 4, 2002, Ser.No. 10/090,669 and now pending SYSTEM AND METHOD FOR PROVIDING NETWORKROUTE REDUNDANCY ACROSS LAYER 2 DEVICES, filed Apr. 16, 2002, Ser. No.10/124,449. The entirety of these applications are hereby incorporatedby reference.

All of the current protocols require devices in a network to beprotocol-aware. That is, each device must be able to run and understandthe protocol that is globally running in the network. A misconfiguredprotocol or malfunctioning device could potentially cause a loop thatwould impact the whole network.

To illustrate this problem, referring to FIG. 5, there is shown anetwork 80 comprising a core or higher priority network such as aprovider 70 coupled to a customer or lower priority network 72 through aswitch 74. Core network 70 runs a conventional spanning tree protocol toavoid loops and has defined a blocked path 76. This means that eitherport 78 or port 80 is blocked. Many different causes may result ininvoluntary loops which may collapse the entire network 80 including:STP corrupted BPDUs, unidirectional optical fibers which result, forexample, when paths which typically comprise two fibers but one has shutdown, and non-configured protocols in loop topologies. In the example inFIG. 5, someone in customer network 72 has improperly disabled the STPrunning in network 72 or, the STP has become disabled due to problemsjust mentioned. As a consequence, even though core network 70 isproperly running the STP to avoid loops, since the customer in network72 is not running the STP, a loop is created in customer network 72 andpackets from customer network 72 flood core network 70. As core network70 and customer network 72 share the same data domain, core network 70will be flooded with customer packets and will be affected adversely bythe customer's action. Yet, it is not possible to ensure that allnetwork administrators or devices are properly doing their respectivejobs and running respective STPs.

Therefore, there is a need in the art for a system and method which candetect and isolate remote loops created in another network.

SUMMARY OF THE INVENTION

Systems and methods are described for enabling a first network to detecta loop in a second network connected thereto. The first network runs afirst instance of a Spanning Tree Protocol and the second network runseither a different instance or no instance. The method includes sendinga Remote Loop Detection Packet (“RLDP”) from the ports in bridges of thefirst network which are connected to the second network. The RLDPincludes identifiers such as the source bridge, port and VLAN. Thesystem and method further includes checking for receipt of the RLDP onthe same bridge which sent the RLDP. If such a receipt occurs, a loop isdetected and one of the ports of the receiving/sending bridge isblocked.

In one aspect of the invention, a method enables a first network todetect a loop in a second network. The second network is connected tothe first network. The first network is running a first loop avoidanceprotocol such a STP. The second network is either running a differentinstance of a loop avoidance protocol or not running any protocol atall.

The method includes sending a first loop packet from a first port in abridge running a loop avoidance protocol of the first network. The firstloop packet includes a first identifier with a first reference to thefirst port. The method further includes receiving a second loop packetat the bridge, the second loop packet including a second identifier witha second reference to a second port. The method still further includesdecoding the second loop packet to determine the second reference,comparing the second reference with the first reference, and detectingthe loop in the second network when the first and second referencesmatch.

In another aspect of the invention a system enables a first network todetect a loop in a second network. The second network is communicablycoupled to the first network. The first network is running a first loopavoidance protocol instance, the second network is not running the firstloop avoidance protocol instance. The system comprises a first network,a bridge in the first network; and a first port in the bridge. The firstport sends a first loop packet including a first identifier with a firstreference to the first port. The bridge receives a second loop packet,the second loop packet including a second identifier with a secondreference to a second port. The bridge further determines the secondreference, compares the second reference with the first reference, anddetects the loop in the second network when the first and secondreferences match.

In yet another aspect of the invention, a bridge in a first network iscommunicably coupled to a second network. The first network is running afirst loop avoidance protocol instance. The second network is notrunning the first loop avoidance protocol instance. The bridge comprisesa first port. The first port sends a first loop packet including a firstidentifier with a first reference to the first port. The bridge receivesa second loop packet, the second loop packet including a secondidentifier with a second reference to a second port. The bridge furtherdetermines the second reference, compares the second reference with thefirst reference, and detects a loop in a second network when the firstand second references match.

In still yet another aspect of the invention, a computer readablestorage medium includes computer executable code for enabling a firstnetwork to detect a loop in a second network. The second network iscommunicably coupled to the first network. The first network is runninga first loop avoidance protocol instance. The second network is notrunning the first loop avoidance protocol instance. The code performsthe steps of sending a first loop packet from a first port in a bridgeof the first network, the first loop packet including a first identifierwith a first reference to the first port. The code further performsreceiving a second loop packet at the bridge, the second loop packetincluding a second identifier with a second reference to a second port.The code determines the second reference, compares the second referencewith the first reference and detects the loop in the second network whenthe first and second references match.

In yet another aspect of the invention, a system enables a first networkto detect a loop in a second network. The second network beingcommunicably coupled to the first network. The first network running afirst loop avoidance protocol instance, the second network not runningthe first loop avoidance protocol instance. The system comprises a firstnetwork and a plurality of bridges in the first network. The systemfurther comprises a plurality of ports, at least one port for each ofthe bridges. Each port connected to the second network sends arespective first loop packet including a first identifier with a firstreference to the respective port. Each bridge receives a respectivesecond loop packet, each second loop packet including a respectivesecond identifier with a respective second reference to a respectivesecond port. Each respective bridge further determines the respectivesecond reference, compares the respective second reference with therespective first reference, and detects a loop in the second networkwhen the respective first and respective second references match.

Still yet another aspect of the invention is a method for enabling afirst network to detect a loop in a second network communicably coupledto the first network. The first network is running a first loopavoidance protocol instance. The second network is not running the firstloop avoidance protocol instance. The method comprises running a secondprotocol in the first network to detect a loop in the second network andprotecting the first network when a loop is detected in the secondnetwork.

Yet still another aspect of the invention is a system for enabling afirst network to detect a loop in a second network communicably coupledto the first network. The system comprises a first network running afirst loop avoidance protocol instance. A second network is not runningthe first loop avoidance protocol instance. The first network runs asecond loop avoidance protocol instance to detect for a loop in thesecond network. The first network further protects the first networkwhen a loop is detected in the second network.

Still yet another aspect of the invention is a system comprising a firstnetwork running a first loop avoidance protocol instance. A secondnetwork is communicably coupled to the first network. The second networkis not running the first loop avoidance protocol instance and has aloop. The first network is protected from the loop in the secondnetwork.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 are network diagrams showing prior art network architecture.

FIG. 5 is a network diagram showing a prior art network architecturewhere an undesired loop has formed.

FIG. 6 is a network diagram detailing some of the functioning of oneembodiment of the invention.

FIG. 7 is a network diagram showing a blown up view of a portion of FIG.6 and detailing some of the functioning of one embodiment of theinvention.

FIG. 8 is a network diagram detailing some of the functioning of oneembodiment of the invention.

FIG. 9 is a flow chart detailing some of the functioning of theinvention.

FIG. 9 a is a flow chart detailing some of the functioning of theinvention.

FIG. 10 is a schematic diagram showing an example of some of thecomponents of a switch in accordance with the invention along with anexample of recording media.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As stated above, it is not possible to assure that all networkadministrators adhere to their task of running a STP or that all networkdevices operate properly. It is therefore desirable to be able toisolate a first network from other networks coupled thereto in case aloop occurs. For example, in L2 metro provider cases, a network in SanJose should not be brought down because a network administrator in SanFrancisco forgot to enable STP or other loop avoidance protocol on hisswitches or because a device or other failure in San Francisco causedSTP.

Referring now to FIG. 6, in accordance with the invention, a Remote LoopDetection Protocol (“RLDP”) is established. The RLDP is a port-VLANoriented protocol or program used to detect loops in a network 100. TheRLDP may be run out of every port in every VLAN coupled to anothernetwork. The protocol is light and should not cause high CPUutilization. For example, core network 102 may be running a firstinstance of a STP while the connected networks may be running adifferent instance or no instance.

The RLDP allows for any switch in network 100 to remotely monitor anynetwork connected to its ports. Upon detection of a loop in the remotenetwork, the RLDP takes administrative action (discussed below) to blockports connected to the remote network with the loop. As shown in FIG. 6,with core network 102 communicably coupled to customer networks 110,112, 114, 116 and 118, RLDP is enabled in switches 104, 106 and 108 butnot necessarily in switch 107. Although core network 102 is showndirectly coupled to customer networks 110, 112, 114, 116 and 118,clearly these networks may also be indirectly communicably coupledthrough other intervening networks. Additionally, networks 110, 112,114, 116 and 118 may choose to run RLDP in switches 124, 126, 128, 130,132, 134 and 136 respectively. For the purposes of illustration thefollowing discussion will focus on core network 102 using RLDP to detecta loop in a network connected to it.

Switch 104 is shown in a blow up 120 in FIG. 6 illustrating the presenceof the RLDP software module 122 which is included in switch 104, switch106 and switch 108. The RLDP program may be stored in switch 104 or maybe stored remotely and accessed by switch 104. With respect to switch104, when a loop is detected in a particular one of customer networks110, 112, 114, 116 or 118, that particular customer network is isolatedfrom core network 102 while the remaining customer networks may remainconnected to core network 102.

When the RLDP is enabled on a port of a switch, that port generates RLDPpackets which are sent out at a constant interval—for example 0.1seconds—which may be changed by the operator. The RLDP packets includeunique information discussed below. The packets are L2 multicast packetswith a MAC address of 0x030480000102. The packets are sent from ports ina VLAN where the RLDP is enabled and follow the tag mode of theparticular port. If the port is tagged to a VLAN, an IEEE 802.1Q tag isadded to the packet between the Media Control Access (“MAC”) address andthe data portion of the packet.

Exemplary contents of a RLDP packet are shown immediately below:

Protocol Identifier—2 bytes. This is encoded in the first two octets ofthe RLDP packet and takes the value of “1”. Protocol Version—1 byte.This is encoded in the third octet and takes the value of “0”. VLANIdentifier—2 bytes. This is encoded in the fourth and fifth octet andtakes the value of the VLAN where the RLDP packet originated. Bridgeidentifier—6 bytes. This is encoded in octets 6 through 11. Itrepresents the bridge identification which should be unique. The firstMAC address of the bridge may be used. Port Identifier—2 bytes. This isencoded in the twelfth and thirteenth octet. It includes the port ID inthe system and should be a unique number within the bridge. The SNMP(Simple Network Management Protocol) interface ID may be used.

Referring now to FIG. 7, when a RLDP packet is received on a port of aswitch running the RLDP, such as switch 108, the RLDP determines whetherthe bridge identifier and port identifier of the received packetcorresponds to the bridge/switch which received the received packet. Ifthe identifiers do match, the RLDP has detected a loop in remote network118 and action is taken to isolate that loop and the network. In orderfor a match to occur, the RLDP packet would have to originate in thereceiving bridge, travel in a loop, and the then return to the receivingbridge. As network 102 has a blocked path, the loop must be in thecustomer network 118 attached to it.

The action taken by the RLDP includes blocking a data path either on theport 144 sending the RLDP packet or the port 142 that received the RLDPpacket. The default option is that the RLDP will block the port thatreceives the RLDP packets. Such a situation is shown in FIG. 7. However,as shown in FIG. 8, sometimes blocking the receiving port is notdesirable as such blocking may impact all of network 102. In thissituation, the sending port 146 of bridge 106 is blocked. Generally,each network administrator decides, based on the architecture of thenetwork, which ports to be blocked when a loop is found.

However, referring again to FIG. 6, if a customer network is connectedto the provider network 102 through two ports, both of which are runningthe RLDP, as is the case with switch 104, a different procedure is used.As both ports 104 a and 104 b are sending out RLDP packets, if a loop isdetected in network 110, both ports will receive these packets and willmove to a blocking state. To avoid this situation, as an alternativeembodiment, if a loop is detected, the RLDP determines whether the portwhich received the packet is different from the port which sent thepacket. If they are the not different, then the sending/receiving portis blocked. If they are different, then if the receiving port has alower port ID than the sending port, then the receiving port is blocked.Otherwise, the sending port is blocked. Of course, the port with ahigher ID could be blocked or any other method used which ensures thatone port is blocked even if more than one port receives a RLDP packetindicating a loop.

The RLDP software continues to send and receive RLDP packets on portsthat are in the blocking state. No other data is received because theport is in a blocked state. However, the RLDP packets are still receivedso that the switch knows when the loop is fixed. Continuing with theexample shown in FIG. 7, if the RLDP packet corresponding to port 142 isno longer received on port 142, it is likely that the loop is fixed.Thus, if a RLDP packet is not received in a known loop for a per portwaiting time, port 142 changed from blocking to forwarding. The per portwaiting time can be configured and its default value is 10 seconds.

Referring now to FIG. 9, there is shown a flow chart summarizing theoperations of the invention for a particular bridge/switch operating theRLDP. At step S2, the RLDP software queries whether it is time to send aRLDP packet. If it is time, the packet is sent at step S3 and controlbranches to Step S5. If not, control still branches to step S5 where theRLDP software queries whether a RLDP packet has been received. If such apacket has been received, control branches to step S4. If not, controlstill branches to step S24 where the RLDP software queries whether anyport is blocked. If the answer is yes, the control branches to step S26.If the answer in step S24 is no, control branches back to step S1.

Assuming that a RLDP packet has been received, at step S4, the RLDPdecodes the bridge identifier received in the packet. At step S6, theRLDP determines whether the bridge identifier in the received RLDPpacket matches the bridge identifier of the particular bridge. If theidentifiers do not match, the frame in the RLDP packet is flooded to theapplicable ports in the VLAN in step S14 and control branches back tostep S1. If the bridge identifiers do match, control branch to step S8where the VLAN and port IDs are decoded from the received RLDP packet.

Control then branches to step S10, where the RLDP software determineswhether the RLDP program is running on the decoded port and VLAN. If theprogram is not running, control branches to step S12 where the frame isdropped because presumably there is a log error and then controlbranches back to step S1. If the program is running on the decoded portand VLAN, control branches to step S16 where the RLDP softwaredetermines whether the block receive mode is enabled. The block receivemode dictates whether the port sending RLDP packets or the port whichreceived the RLDP packet should be blocked. If this mode is enabled,control branches to step S21 where the RLDP determines whether thereceiving port is already blocked. If it is, control branches to stepS1. If not, control branches to step S22 and the port which received theRLDP packet is blocked. If the blocking mode is not enabled at step S16,control branches to step S19 where the RLDP determines whether the portwhose ID is in the received RLDP packet is blocked. If it is, controlbranches to step S1. If not, control branches to step S18 where the portwhose ID is in the received RLDP packet is blocked. After either stepsS18 or S22, control branches to step S20 where the current time ismarked as the last time a RLDP packet was received and control branchesback to S1.

Referring back to step S24, where the RLDP determines whether any portis blocked. If no port is blocked, control branches back to step S1. Ifa port is blocked, control branches to step S26 where the RLDP querieswhether the current time minus the last time a RLDP packet was receivedis greater than or equal to the per port waiting time for the blockedport. If the answer is no, control branches to step S1. If the answer isyes, control branches to step S28 and the blocking port is set to aforwarding port and then control branches back to step S1.

Referring to FIG. 9A, there is shown another flow chart summarizing someof the features of the invention. As stated above, if a customer networkis connected to two ports, both running a RLDP, if a loop is detected inthe customer network, both ports may end up in a blocking state. Inaddition to determining which port to block using step S16, the RLDP mayinclude steps S11 and S15 as shown in FIG. 9A. As in the priorembodiment, if a RLDP packet is received, control branches through stepsS5, S4, S6, S8 and S10 as discussed above. If the answer to the query instep S10 is yes, control branches to step S11 where the RLDP determineswhether the RLDP program is running on the received port. If the answerat step S11 is no, control branches to step S16 as discussed above.

If the answer to the query at step S11 is yes, then control branches tostep S15 where the RLDP queries whether the receiving port ID is lessthan or equal to the sending port ID. If the answer is yes, controlbranches to step S19 as discussed above. If the answer is no, controlbranches to step S21 as discussed above. Clearly, the decision made instep S15 could be effectuated using the port with a higher ID or anyother method which ensures that one port is blocked even if more thanone port receives a RLDP packet indicating a loop. Whatever method ischosen, such method will override any customer configuration.

Referring to FIG. 10, each switch may comprise a conventional computer206 including a CPU 200, a read only memory (“ROM”) 202, a random accessmemory (“RAM”) 204, a storage device 208, a network interface (such asthe ports discussed above) 210 and an input device 212 all coupledtogether by a bus 214. The RLDP program may be stored on computer 206,on storage media 216 or stored remotely.

Thus, by broadcasting a unique packet from each port which includes anidentifier of that port, determining whether packets received at aparticular port include the identifier for the port, and blocking portsbased on this determination, a system and method for isolating remoteloops is achieved.

While the invention has been described and illustrated in connectionwith preferred embodiments, many variations and modifications as will beevident to those skilled in this art may be made without departing fromthe spirit and scope of the invention, and the invention is thus not tobe limited to the precise details of methodology or construction setforth above as such variations and modification are intended to beincluded within the scope of the invention.

1. A method comprising: at a network device comprising a memory andconfigured to perform packet switching, storing in the memory a firstloop packet for sending from a first port of the network device, thenetwork device configured to interface with a first network running aloop avoidance protocol instance, the first loop packet including afirst identifier with a first reference to the first port; responsive tothe sending, examining a second loop packet, the second loop packetincluding a second identifier with a second reference to a second port;and if the first reference and the second reference match, indicatingdetection of a loop in a second network communicably coupled to thefirst network and running at least one loop avoidance protocol instanceother than the first loop avoidance protocol instance.
 2. The method ofclaim 1, further comprising blocking at least one of the ports in thenetwork device when the first and second references match.
 3. The methodof claim 2, wherein the blocking includes blocking the port which sentthe first loop packet.
 4. The method of claim 2, wherein the blockingincludes blocking a port which received the second loop packet.
 5. Themethod of claim 2, further comprising, when the second reference doesnot match the first reference, for a defined period of time, opening theat least one of the ports in the network device which was blocked. 6.The method of claim 1, further comprising when the second identifierdoes not correspond to any of the ports in the network device,forwarding the second loop packet to another network device.
 7. Themethod of claim 1, wherein the first identifier further includes a thirdreference to the network device, the second identifier further includesa fourth reference to another network device, and the method furthercomprises: comparing the third and fourth references; and when the thirdreference matches the fourth reference and the first reference does notmatch the second reference, dropping the second loop packet.
 8. Themethod of claim 1, wherein the first network is running a first instanceof a spanning tree protocol and the second network is not running thefirst instance of the spanning tree protocol.
 9. The method of claim 1,wherein the first identifier includes a third reference to a VLAN wherethe first port resides.
 10. The method of claim 2, wherein the blockingincludes blocking one of the port which sent the first loop packet and aport which received the second loop packet based on the respective portIDs.
 11. A network device comprising: a memory; and one or more modulesconfigured to: store in the memory a first loop packet for sending froma first port of the network device, the network device configured toperform packet switching and to interface with a first network running aloop avoidance protocol instance, the first loop packet including afirst identifier with a first reference to the first port; responsive tothe sending, examine a second loop packet, the second loop packetincluding a second identifier with a second reference to a second port;and if the first reference and the second reference match, indicatedetection of a loop in a second network communicably coupled to thefirst network and running at least one loop avoidance protocol instanceother than the first loop avoidance protocol instance.
 12. The networkdevice of claim 11 wherein the one or more modules are furtherconfigured to block at least one of the ports in the network device whenthe first and second references match.
 13. The network device of claim12 wherein the blocking includes blocking the port which sent the firstloop packet.
 14. The network device of claim 12 wherein the blockingincludes blocking a port which received the second loop packet.
 15. Thenetwork device of claim 12 wherein the one or more modules are furtherconfigured to, when the second reference does not match the firstreference, for a defined period of time, open the at least one of theports in the network device which was blocked.
 16. The network device ofclaim 11 wherein the one or more modules are further configured to, whenthe second identifier does not correspond to any of the ports in thenetwork device, forward the second loop packet to another networkdevice.
 17. The network device of claim 11, wherein the first identifierfurther includes a third reference to the network device, the secondidentifier further includes a fourth reference to another networkdevice, and the one or more modules are further configured to: comparethe third and fourth references; and when the third reference matchesthe fourth reference and the first reference does not match the secondreference, drop the second loop packet.
 18. The network device of claim11, wherein the first network is running a first instance of a spanningtree protocol and the second network is not running the first instanceof the spanning tree protocol.
 19. The network device of claim 11,wherein the first identifier includes a third reference to a VLAN wherethe first port resides.
 20. The network device of claim 12, wherein theone or more modules are further configured to block one of the portwhich sent the first loop packet and a port which received the secondloop packet based on the respective port IDs.
 21. A nontransitoryprogram storage device readable by a machine, embodying a program ofinstructions executable by the machine to perform a method, methodcomprising: at a network device comprising a memory and configured toperform packet switching, storing in the memory a first loop packet forsending from a first port of the network device, the network deviceconfigured to interface with a first network running a loop avoidanceprotocol instance, the first loop packet including a first identifierwith a first reference to the first port; responsive to the sending,examining a second loop packet, the second loop packet including asecond identifier with a second reference to a second port; and if thefirst reference and the second reference match, indicating detection ofa loop in a second network communicably coupled to the first network andrunning at least one loop avoidance protocol instance other than thefirst loop avoidance protocol instance.
 22. An apparatus comprising:means for, at a network device comprising a memory and configured toperform packet switching, storing in the memory a first loop packet forsending from a first port of the network device, the network deviceconfigured to interface with a first network running a loop avoidanceprotocol instance, the first loop packet including a first identifierwith a first reference to the first port; means for, responsive to thesending, examining a second loop packet, the second loop packetincluding a second identifier with a second reference to a second port;and means for, if the first reference and the second reference match,indicating detection of a loop in a second network communicably coupledto the first network and running at least one loop avoidance protocolinstance other than the first loop avoidance protocol instance.